The present invention relates to a plating solution for palladium plating, and especially to a palladium plating film as materials for electric and electronic parts required to have solder wettability and heat resistance, a palladium plating solution for obtaining the film, and a lead frame for semiconductor apparatuses having the palladium plating film formed using the plating solution.
Palladium plating solutions have been widely studied for a long time, and the plating films obtained therefrom have characteristics such as excellent corrosion resistance, wear resistance, and electric properties. Therefore, palladium plating has been used as a substitute for gold plating in uses relating to electrical and electronic parts such as electrical contacts, connectors, and circuit substrates, since it is more economical than gold plating.
Various palladium plating solutions have been developed, such as, for example, those comprising palladium chlorides and pyridine, sulfates, and nitrites. However, when these palladium plating solutions are used, internal stress in the plating films tends to increase, and, hence, either control of thickness is difficult, or no sufficient ductility can be obtained. Furthermore, with regard to the practical problems, e.g., insufficient bonding property and solder wettability after heat treatment which are important as plating for electronic parts, there are known palladium plating solutions which contain selenium, cerium, urea or fluorine-based surface active agent for solving the problem of internal stress or improving glossiness.
Moreover, there are known plating films having on the surface Pd or Pd-Ni alloy film (JP-B-63-49382), Pd-Bi alloy film (JP-A-4-287089), Pd-X (X=boron, phosphorus, arsenic, selenium, tellurium, antimony: JP-A-6-232311), or having an Au plating film on Pd or Pd alloy film (JP-A-4-115558).
However, these conventional techniques do not sufficiently solve the practical problems such as solder wettability, heat resistance and bonding property. Recently, the performance of various electronic apparatuses has been further enhanced and more severe demands have been made for semiconductor devices. Therefore, palladium plating materials used in these applications must have the properties of higher performance and stability, and thus demand for improvement of palladium plating solutions becomes stronger.
For example, palladium plating solutions are also utilized in the field of lead frames which are components for mounting chips of semiconductor devices, but there remain problems to be solved as stated below.
FIG. 4 is a plan view which shows a general example of a lead frame, and FIG. 5 is a sectional view of the lead frame shown in FIG. 4. In FIG. 4 and FIG. 5, inner lead part 2 is formed around chip-mounting part 1, and outer lead part 3 is provided through tie bar part 4. Semiconductor chip 5 is mounted on the chip-mounting part 1 and fixed by adhesive 6, and, furthermore, electrode 7 on the semiconductor chip 5 and the inner lead part 2 are bonded by wire 8 and besides sealed by sealing resin 9.
In the production of the lead frame shown, the following steps are conducted in succession: the whole of the Ni plated substrate is further subjected to palladium plating; semiconductor chip 5 is bonded to chip-mounting part 1; the terminal of semiconductor chip 5 and inner lead part 2 are bonded by wire 8; and furthermore the whole of the lead frame other than the outer lead part 3 is sealed with sealing resin 9.
The plan view of the lead frame shown in FIG. 4 illustrates only the lead frame before bonding of semiconductor chip 5, bonding of the chip and the inner lead frame part by wire 8 and sealing with sealing resin 9. After bonding of semiconductor chip 5, bonding by wire 8 and sealing with sealing resin 9, the frame lead is cut so as to attain such state that the corresponding inner lead part 2 and outer lead part 3 are electrically connected and adjacent inner lead parts 2 and adjacent outer lead parts 3 do not short-circuit. Thereafter, the outer lead part 3 is bonded to other electronic parts with solder, and used.
Palladium is a noble metal which is a chemically stable material like gold. There are, therefore no special problems in the bonding property of palladium plating film to semiconductor chip 5 and in wire bonding property. However, the palladium plating film undergoes an oxidation reaction by the heating to high temperatures at the time of bonding of semiconductor chip 5 to the palladium plating film, resulting in deterioration of solder wettability in the subsequent steps. This causes difficulty in soldering outer lead part 3 which is exposed outside after sealing with sealing resin 9. That is, the wetting rate lowers and it becomes difficult to cover the required solder wetting area. For this reason, the dipping time in the soldering tank increases and results in deterioration of operation efficiency, and, in an extreme case, the solder cannot be applied at all. Especially, with the recent increase in density of semiconductor devices, the distance between outer lead parts 3 becomes smaller, and,at the time of reflow bonding with a cream solder in surface assembling, the cream solder flows in transverse direction of substrate due to insufficient wettability of solder. Bridges formed between wirings can cause short short circuits. Formation of the bridge can be diminished to some extent by reducing the coating amount of the cream solder, while due to reduction of coating amount of cream solder, coating height of cream solder lowers and sometimes bonding becomes impossible owing to variation of distance between outer lead part 3 and substrate, resulting in an opening of the circuit.
The present invention solves the above problems in conventional techniques and aims at providing a palladium plating solution for obtaining a palladium plating film having excellent soldering characteristics such as good solder wettability and high solder wetting rate, which are not deteriorated in a high-temperature atmosphere, and furthermore providing such a plating film.